I think that the brain has a desperate need to fill in gaps in knowledge.
It acts something like the visual processors of the brain, which scream in desperation from lack of input when we fall unconscious.. and fabricate dreams.
It's as though their imagination desperately reaches for any answer, even answers outside of the normal requirements for belief -- evidence, logic and otherwise an otherwise complete chain of trust.
The need to fill the gap in knowledge creates credulity to the point of gullibility. This is like the need for stimuli when sleeping creates dreams.
It's possible to be dominant with one hand or the other. It's possible to be dominant with one eye or the other. I'd bet it's possible to be dominant with one part of the brain or another.
I'm more and more leaning toward the idea that very religious people are biologically different.
It's certain that there are sliding scales of personality, most if not all of which have a biological basis, which brush against genius. A similar biological sliding scale of something like credulity doesn't seem outlandish to me.